The cause of the subsistence crisis in France was:
- The population of France rose from about 23 million in 1715 to 28 million 1789. This led to a sudden increase in the demand for foodgrains.
 - Production of grains could not keep pace with the demand. So, the price of bread which was the staple diet of the majority rose rapidly.
 - Most workers were employed as labourers in workshops whose owner fixed their wages. However, their wages did not keep pace with the rise in prices.
 - So, the gap between the poor and the rich widened.
 - Things became worse whenever drought or hail reduced the harvest 
 - This led to a subsistence crisis, something that occured frequently in France during the Ancien Regime.
 
Hope you could get an idea from here.
Doubt clarification - use comment section